System and method for proximity detection with single-antenna device

ABSTRACT

A single-antenna device includes a single antenna, at least one processor, and at least one memory. The single-antenna device is operable to receive a signal including at least one frame. Each of said frame includes a repeating portion. The single-antenna device determines a difference of phase and amplitude of the repeating portion and further determines whether the signal is transmitted from a trusted source based at least in part on the difference of phase and amplitude of the repeating portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a National Stage Entry of InternationalPatent Application No. PCT/US2019/029395, filed on Apr. 26, 2019, whichclaims priority to U.S. Provisional Patent Application No. 62/663,543,filed on Apr. 27, 2018. The entire contents of each of theaforementioned applications are fully incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under CNS-1329686awarded by the National Science Foundation. The government has certainrights in the invention.

TECHNICAL FIELD

The present invention relates to a system and method for proximitydetection with a single-antenna device and, in particular, a system andmethod for detecting proximity between a transmitting device and asingle-antenna device based on a repeating portion of a wireless signalreceived at the single-antenna device.

BACKGROUND

Billions of Internet of Things (IoT) devices are envisioned to bedeployed in the near future, with new devices entering and exiting localenvironments in an unpredictable manner. These devices are projected tocollect and share data among each other, some of which may be privacysensitive or have security implications.

Securely transferring data between two devices that have not previouslyshared a secret is a difficult task. Previous solutions such asDiffie-Hellman key exchange are subject to well-known Man-in-The-Middleattacks. Other solutions such as Public Key Infrastructure requiresupport from trusted servers on the Internet. Furthermore, as the numberof wireless devices grows, manually configuring each device tocommunicate with its neighbors becomes increasingly impractical. Indeed,manually entering secret keys on each device will likely becomeextraordinarily cumbersome if predictions of the number of IoT devicescoming soon are even remotely accurate.

Therefore, there is a need for new methods and systems to facilitatereliable and secure communication between two devices in close physicalproximity, even when those devices have never met nor shared a key.

U.S. Provisional Application No. 62/554,867, filed on Sep. 6, 2017provides a method for secure short-range information exchange between amulti-antenna device and a target device. However, a single-antennadevice cannot use a multi-antenna-based method and, therefore, has noway to verify its proximity to the target device. These single-antennadevices, such as IoT devices, may be vulnerable to attack.

SUMMARY OF THE INVENTION

The present disclosure provides methods and systems for securelytransferring data between a single-antenna device and a transmittingdevice.

In one aspect, this disclosure provides a method for establishing trustbetween a single-antenna device and a transmitting device. The methodcomprises placing the single-antenna device and the transmitting devicein close physical proximity to each other. In certain embodiments, thesingle-antenna device and the transmitting device are placed less thanten centimeters apart from each other, alternatively less than less thannine centimeters apart from each other, alternatively less than eightcentimeters apart from each other, alternatively less than sevencentimeters apart from each other, alternatively less than sixcentimeters apart from each other, alternatively less than fivecentimeters apart from each other, alternatively less than fourcentimeters apart from each other, alternatively less than threecentimeters apart from each other, alternatively less than twocentimeters apart from each other, or alternatively less than onecentimeter apart from each other. In some such embodiments, thesingle-antenna device and the transmitting device are about onecentimeter, about two centimeters, about three centimeters, about fourcentimeters, about five centimeters, about six centimeters, about sevencentimeters, about eight centimeters, or about nine centimeters apartfrom each other. In certain embodiments, the method further comprisestransmitting a preamble from the transmitting device. In some suchembodiments, the preamble may be a Wi-Fi preamble. In some suchembodiments, the preamble comprises at least one long training field(LTF). In some such embodiments, the preamble comprises a repeatingportion, such as a repeating portion of the LTF in the Wi-Fispecification (IEEE 802.11). In some such embodiments, thesingle-antenna device identifies the transmitting device as a trustedtransmitting device using the repeating portion of the preamble. Forexample, when the single-antenna device is physically close to thetransmitting device, near-field effects will cause the repeatingportions of the preamble to differ in phase and amplitude, and thesingle-antenna device may use this information to identify thetransmitting device as a trusted transmitting device based on itsproximity. In certain embodiments, the method is keyless (i.e., themethod does not comprise sharing a key between the single-antenna deviceand the transmitting device).

In one aspect, this disclosure provides a method for determiningproximity between a single-antenna device and a transmitting device. Incertain embodiments, the single-antenna device is configured to receivea preamble from a transmitting device. In certain embodiments, themethod comprises receiving, by the single-antenna device, a preambletransmitted from the transmitting device. In some such embodiments, thepreamble comprises at least one long training field (LTF). In some suchembodiments, the preamble comprises a repeating portion, such as arepeating portion of the LTF in the Wi-Fi specification (IEEE 802.11).In some such embodiments, the single-antenna device determines theproximity of the transmitting device using the repeating portion of thepreamble. For example, when the single-antenna device is physicallyclose to the transmitting device, near-field effects will cause therepeating portions of the preamble to differ in phase and amplitude. Onthe other hand, when the single-antenna device is remote from thetransmitting device (e.g., more than about ten centimeters away), therepeating portions of the preamble will be received with a consistent orsubstantially consistent phase and amplitude. The single-antenna devicemay use this information to determine the proximity of the transmittingdevice (e.g., within about 10 centimeters from the single-antenna deviceor more remote from the single-antenna device). In certain embodiments,the method is keyless (i.e., the method does not comprise sharing a keybetween the single-antenna device and the transmitting device).

In one aspect, this disclosure provides a method for determining whethera radio signal originated with a target transmitting device or apotentially adversarial transmitting device. In certain embodiments, areceiving device, such as a single-antenna device, uses the phase and/oramplitude of a preamble received from a transmitting device,particularly a repeating portion of the preamble (e.g., repeatingportions of the LTF), to determine whether the receiving device is inclose proximity to the transmitting device. For example, when asingle-antenna device is physically close to the transmitting device,near-field effects will cause repeating portions of the preamble todiffer in phase and amplitude. On the other hand, when thesingle-antenna device is far from the transmitting device, such as apotential adversary, the repeating portions of the preamble will bereceived with a consistent or substantially consistent phase andamplitude. In some such embodiments, the single-antenna deviceidentifies a transmitting device as a legitimate device when the phaseand/or amplitude of the repeating portions of the preamble received fromthe transmitting device are different. In other such embodiments, thesingle-antenna device identifies a transmitting device as a potentialadversary when the phase and/or amplitude of the repeating portions ofthe preamble received from the transmitting device are consistent orsubstantially consistent. In certain embodiments, the method is keyless(i.e., the method does not comprise sharing a key between thesingle-antenna device and the transmitting device).

In one aspect, this disclosure provides a system for secure short-rangeinformation exchange. The system comprises a single-antenna devicecomprising an antenna, ATARG, configured to receive wireless data,wherein antenna ATARG is located within about ten centimeters,alternatively within about nine centimeters, alternatively within abouteight centimeters, alternatively within about seven centimeters,alternatively within about six centimeters, alternatively within aboutfive centimeters, alternatively within about four centimeters,alternatively within about three centimeters, alternatively within abouttwo centimeters, or alternatively within about one centimeter from atransmitting device. In certain embodiments, the transmitting device isconfigured to transmit a preamble to the single-antenna device. Incertain embodiments, the single-antenna device is configured to receivea preamble from the transmitting device. In some such embodiments, thepreamble comprises at least one long training field (LTF). In some suchembodiments, the preamble comprises a repeating portion, such as arepeating portion of the LTF in the Wi-Fi specification. In certainembodiments, the single-antenna device is configured to determineproximity with the transmitting device using the preamble, particularlyrepeating portions of the preamble, received from the transmittingdevice.

In certain embodiments for any of the aspects described herein, therepeating portion of the preamble comprises a first set of symbols and asecond set of symbols, wherein the second set of symbols is identical orsubstantially identical of the first set of symbols. In some suchembodiments, the first set of symbols and the second set of symbols areorthogonal frequency division multiplexing (OFDM) symbols. In some suchembodiments, each of the first set of symbols and the second set ofsymbols is 64-sample OFDM symbols.

In certain embodiments for any of the aspects described herein, thesingle-antenna device is a wireless device. In certain embodiments forany of the aspects described herein, the single-antenna device is amobile device, an Internet of Things (IoT) type device, a personalcomputer (PC), a medical device, a household appliance, a wearabledevice, a vehicle (e.g., automobile, aircraft) component, or the like.

In certain embodiments for any of the aspects described herein, themethod further comprises assessing signal strength of a preamble. Insome such embodiments, signal strength is assessed if the repeatingportion of the preamble is determined to be different. In some suchembodiments, signal strength is assessed as a check to prevent a distantadversary from tricking the single-antenna-device into believing that amalformed preamble is a legitimate signal from a nearby device. Forexample, if the single-antenna device detects a low strength signal, itdetermines that the signal came from a distant transmitting device, andnot from a physically proximate device.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made toembodiments shown in the following drawings. The components in thedrawings are not necessarily to scale and related elements may beomitted, or in some instances proportions may have been exaggerated, soas to emphasize and clearly illustrate the novel features describedherein. In addition, system components can be variously arranged, asknown in the art. Further, in the drawings, like reference numeralsdesignate corresponding parts throughout the several views.

FIG. 1 illustrates an example system comprising a plurality ofcommunication devices.

FIG. 2 illustrates the detailed structure of a Wi-Fi frame.

FIG. 3 illustrates an example graph representing two 64-sample OFDMsymbols in the LTF of a Wi-Fi frame.

FIG. 4 illustrates a plurality of regions surrounding a transmittingantenna.

FIG. 5 illustrates the orientation of a transmitting antenna in a threedimensional space and a signal propagating from the transmittingantenna.

FIG. 6 illustrates an example graph of power of radial and verticalcomponents of a signal transmitted from a transmitter to asingle-antenna device.

FIG. 7 illustrates an example constellation diagram showing a distancebetween Y₁ and Y₂ for a subcarrier.

FIG. 8 illustrates an example constellation diagram showing the distancebetween Y₁ and Y₂ for all subcarriers of one frame.

FIG. 9 illustrates an example distribution graph of preamble deviationsfor 1,000 Wi-Fi frames received from the transmitting antenna.

FIG. 10 illustrates an example graph of average preamble deviations of aplurality of frames transmitted over a plurality of distances for eachantenna type.

FIG. 11 illustrates an example graph of a likelihood of detectingproximity using average preamble deviations.

FIG. 12 illustrates another example system comprising a plurality ofcommunication devices.

FIGS. 13A and 13B illustrate an example flowchart of a method forestablishing secure short-range information exchange between asingle-antenna device and a transmitting device.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

While the invention may be embodied in various forms, there are shown inthe drawings, and will hereinafter be described, some exemplary andnon-limiting embodiments, with the understanding that the presentdisclosure is to be considered an exemplification of the invention andis not intended to limit the invention to the specific embodimentsillustrated.

Mobile IoT devices are ever increasing in modern technology. These IoTdevices are envisioned to share data and provide control informationamong themselves, and some of that information may be privacy sensitiveor have security implications. This situation suggests that devices thathave never met, nor previously established communication, must somehowhave a means to securely communicate that is consistent with userintent.

Securely transferring data between two devices that have not previouslyestablished communication and/or received information indicative of eachother's identity is a difficult task. The main difficulty is that thenewly discovered devices do not have a common point of trust. In thesesituations, using physical proximity as a basis of trust has beenproposed. The idea is that a user can express intent to introduce twodevices by bringing said devices in close proximity, at leasttemporarily, and then taking an action, such as pressing a button. Thephysical proximity between said devices then forms the basis of trust,thus establishing a secure connection among these devices. A distantadversary, however, may attempt to trick a user's device into acceptinga malicious payload by impersonating a nearby legitimate device.

Several techniques have been proposed to combat such impersonationattacks. Often, these techniques rely on short-range out-of-bandcommunication where devices use a secret channel for communication thatis impervious to observation or interference by an adversary. Thesemethods frequently require additional hardware such as accelerometers,light sensors, or specialized radio frequency (RF) devices such asNear-field communication (NFC) devices. The required out-of-bandhardware may not be present on some devices and these approaches oftenrequire complex processing that exceeds the capabilities of manyembedded devices. Other approaches to thwarting distant adversaries usein-band RF but rely on multiple antennas to simultaneously measuresignal strength to determine proximity or to locate a device in threedimensions. Single-antenna IoT devices with limited hardware that followstandard communication protocols, however, cannot use these techniques.

As described herein, a system and method for establishing secureshort-range information exchange between a single-antenna device and atransmitting device comprise: (i) receiving a preamble transmitted fromthe transmitting device, wherein the preamble comprises at least onerepeating portion; and (ii) determining the proximity of thetransmitting device to the single-antenna device and/or identifying thetransmitting device as a trusted transmitting device or a potentiallyadversarial transmitting device based on the phase and/or amplitude ofthe repeating portion of the preamble. In certain embodiments, therepeating portion of the preamble comprises two identical orsubstantially identical orthogonal frequency division multiplexing(OFDM) symbols. In certain embodiments, the repeating portion of thepreamble is defined in a Long Training Field (LTF) of the preamble. Incertain embodiments, the single-antenna device identifies thetransmitting device as a trusted transmitting device when the phaseand/or amplitude of the repeating portions of the preamble received fromthe transmitting device are different. In certain embodiments, thesingle-antenna device identifies the transmitting device as an untrustedor adversarial transmitting device when the phase and/or amplitude ofthe repeating portions of the preamble received from the transmittingdevice are consistent or substantially consistent. In certainembodiments, the single-antenna device identifies the transmittingdevice as being within about ten centimeters, alternatively within aboutnine centimeters, alternatively within about eight centimeters,alternatively within about seven centimeters, alternatively within aboutsix centimeters, alternatively within about five centimeters,alternatively within about four centimeters, alternatively within aboutthree centimeters, alternatively within about two centimeters, oralternatively within about one centimeter of the single-antenna devicewhen the phase and/or amplitude of the repeating portions of thepreamble received from the transmitting device are different. In certainembodiments, the single-antenna device identifies the transmittingdevice as being greater than ten centimeters away from thesingle-antenna device when the phase and/or amplitude of the repeatingportions of the preamble received from the transmitting device areconsistent or substantially consistent. In certain embodiments, thesingle-antenna device: (i) calculates a total Euclidean distance betweenthe phase and/or amplitude of all subcarriers included in the repeatingportion of the preamble; (ii) calculates an average preamble deviationover a number of preambles transmitted by the transmitting device basedon a sum of all the total Euclidean distances of the number ofpreambles; (iii) compares the average preamble deviation over the numberof preambles transmitted by the transmitting device to a threshold; (iv)identifies the transmitting device as a trusted transmitting device whenthe average preamble deviation over the number of preambles transmittedby the transmitting device is greater than the threshold; and (v)identifies the transmitting device as an untrusted or potentiallyadversarial transmitting device when the average preamble deviation overthe number of preambles transmitted by the transmitting device is lowerthan the threshold. In certain embodiments, the single-antenna device,in response to identifying the transmitting device as a potentialtrusted transmitting device, inquires a separate trusted communicationdevice to confirm whether that trusted communication device sees amatching preamble from a signal transmitted by the transmitting device.In certain embodiments, the single-antenna device, in response toidentifying the transmitting device as a potential trusted transmittingdevice, the single-antenna device: (i) measures a signal strength ofeach preamble transmitted from the transmitting device; and (ii)responsive to the signal strength of said Wi-Fi-preamble being lowerthan a threshold, reject said preamble.

FIG. 1 illustrates an example system 100 comprising a plurality ofcommunication devices. The communication devices include asingle-antenna device 110, a transmitter 120, and an adversarialcommunication device 130. The single-antenna device 110 includes a firstantenna 112 and at least one processor 114 and memory 116. The firstantenna 112 is a single-antenna. For example, the single-antenna may bea half wavelength dipole antenna, a quarter wavelength dipole antenna, amicropatch antenna, a planar inverted-F antenna, or any other type ofsingle-antenna. The processor 114 may be any suitable processing deviceor set of processing devices such as, but not limited to: amicroprocessor, a microcontroller-based platform, a suitable integratedcircuit, one or more field programmable gate arrays (FPGAs), and/or oneor more application-specific integrated circuits (ASICs). The memory 116may be volatile memory (e.g., RAM, which can include non-volatile RAM,magnetic RAM, ferroelectric RAM, and any other suitable forms),non-volatile memory, unalterable memory, read-only memory, and/orhigh-capacity storage devices. In some examples, the memory 116 includesmultiple kinds of memory, particularly volatile memory and non-volatilememory. The memory 116 is computer readable media on which one or moresets of instructions, such as the software for operating the methods ofthe present disclosure can be embedded. The instructions may embody oneor more of the methods or logic as described herein. In a particularembodiment, the instructions may reside completely, or at leastpartially, within any one or more of the memory 114, the computerreadable medium, and/or within the processor 116 during execution of theinstructions. The terms “non-transitory computer-readable medium” and“tangible computer-readable medium” should be understood to include asingle medium or multiple media, such as a centralized or distributeddatabase, and/or associated caches and servers that store one or moresets of instructions. The terms “non-transitory computer-readablemedium” and “tangible computer-readable medium” also include anytangible medium that is capable of storing, encoding or carrying a setof instructions for execution by a processor or that cause a system toperform any one or more of the methods or operations disclosed herein.As used herein, the term “tangible computer readable medium” isexpressly defined to include any type of computer readable storagedevice and/or storage disk and to exclude propagating signals.

While not illustrated, each of the transmitter 120 and the adversarialcommunication device 130 may include at least one processor, memory, andantenna.

In certain embodiments, the single-antenna device 110, the transmitter120, and the adversarial communication device 130 may be a mobiledevice, portable personal computer, a tablet, a wearable device, etc.Each of the single-antenna device 110, the transmitter 120, and theadversarial communication device 130 is capable of establishingcommunication with each other and/or other wireless devices via awireless communication protocol. In the illustrated example, thetransmitter 120 is positioned closer to the single-antenna device 110than the adversarial communication device 130. In the illustratedexample, said wireless communication protocol is Wi-Fi. In theillustrated example, the single-antenna device 110 function as areceiver. In the illustrated example, it is assumed that a user wishesto establish secure short-range communication between the single-antennadevice 110 and the transmitter 120, and the adversarial communicationdevice 130 is an untrusted device attempting to intercept saidcommunication. In the illustrated example, it is assumed that thesingle-antenna device 110 and the transmitter 120 have not previouslyestablished communication with each other and are currently unaware ofeach other's identity. The single-antenna device 110 may determinewhether a received signal is provided from a trusted source (e.g., thetransmitter 120) via proximity detection. Herein, a trusted sourcerefers to a legitimate communication device. The proximity detectioninvolves analyzing a repeating portion of the received signal. Herein,the proximity detection will be described with reference to an exemplarycommunication protocol, the exemplary communication protocol is Wi-Fi;however, as described later in this disclosure, proximity detection maybe performed via other communication protocol that includes a repeatingportion. In the example embodiments below, it is assumed that thereceived signal is a Wi-Fi signal comprising at least one Wi-Fi frame.Details of a Wi-Fi frame will be described with reference to FIG. 2below.

FIG. 2 illustrates the detailed structure of a Wi-Fi frame. The Wi-Fiframe is an Orthogonal Frequency Division Multiplexing (OFDM) Wi-Fiframe. The Wi-Fi frame includes a physical (PHY) layer preamble, aSignal Field, and a Wi-Fi frame's data. The Wi-Fi frame begins with thePHY layer preamble to aid in synchronizing the transmitter 120 and thesingle-antenna device 110. The PHY layer preamble includes a ShortTraining Field (STF) and a Long Training Field (LTF). The Wi-Fi framebegins with the STF, followed by the LTF, followed by the Signal Field,and then the Wi-Fi frame's data. The STF includes 10 identical shorttraining symbols (denoted T₁ through T₁₀ in FIG. 2, where each STFsymbol is sampled 16 times, for a total of 160 samples. The STF is usedby the single-antenna device 110 for frame detection, Automatic GainControl (AGC), diversity selection, coarse frequency offset estimation,and rough symbol timing synchronization. The LTF follows the STF and isused by the single-antenna device 110 for fine frequency correction andchannel estimation. The LTF includes a 32-sample guard interval GI2followed by two identical 64-sample OFDM symbols T₁ and T₂ (i.e.,repeating portion). The guard interval together with the two 64-sampleOFDM symbols T₁ and T₂ make a total of 160 samples in the LTF. Detailsof the two 64-sample OFDM symbols T₁ and T₂ will be further describedwith reference to FIG. 3, below. The Signal Field follows the LTF and isencoded with Binary Phase Shift Keying (BPSK). The Signal Field includesinformation indicative of the number of bytes and the encoding schemeused on the Wi-Fi frame's data. The Wi-Fi frame's data comes after theSignal Field. Each OFDM data symbol included in the Wi-Fi frame's dataincludes a 16-sample guard interval (denoted GI in FIG. 2) and 64samples carrying the actual data.

FIG. 3 illustrates an example graph representing two 64-sample OFDMsymbols T₁ and T₂ included in the LTF of the Wi-Fi frame. As mentionedabove, the two 64-sample OFDM symbols T₁ and T₂ are identical. As such,the phase and amplitude of sample i in symbol T₁ matches the phase andamplitude of sample i+64 in T₂, where i=0 . . . 63. The time-domainsamples may be converted into an equivalent frequency-domainrepresentation by taking a Discrete Fourier Transform (DFT). In someexamples, the time-domain samples may be converted into the equivalentfrequency-domain representation by a Fast Fourier Transform (FFT). Wi-Fisingle-antenna device 110 may perform a 64-point FFT over the receivedtime-domain samples to transform the time-domain samples into thefrequency domain. The FFT operation yields 64 complex numbersrepresenting the phase and amplitude of 64 subcarriers, indexed from −32to +31. Table 1 illustrates the two 64-sample OFDM symbols T₁ and T₂represented in the frequency domain.

TABLE 1 ## Re Im ## Re Im −32 0.000 0.000  0   0.000 0.000 −31 0.0000.000  1   1.000 0.000 −30 0.000 0.000  2 −1.000 0.000 −19 0.000 0.000 3 −1.000 0.000 −28 0.000 0.000  4   1.000 0.000 −27 0.000 0.000  5  1.000 0.000 −26 1.000 0.000  6 −1.000 0.000 −25 1.000 0.000  7   1.0000.000 −24 −1.000   0.000  8 −1.000 0.000 −23 −1.000   0.000  9   1.0000.000 −22 1.000 0.000 10 −1.000 0.000 −21 1.000 0.000 11 −1.000 0.000−20 −1.000   0.000 12 −1.000 0.000 −19 1.000 0.000 13 −1.000 0.000 −18−1.000   0.000 14 −1.000 0.000 −17 1.000 0.000 15   1.000 0.000 −161.000 0.000 16   1.000 0.000 −15 1.000 0.000 17 −1.000 0.000 −14 1.0000.000 18 −1.000 0.000 −13 1.000 0.000 19   1.000 0.000 −12 1.000 0.00020 −1.000 0.000 −11 −1.000   0.000 21   1.000 0.000 −10 −1.000   0.00022 −1.000 0.000  −9 1.000 0.000 23   1.000 0.000  −8 1.000 0.000 24  1.000 0.000  −7 −1.000   0.000 25   1.000 0.000  −6 1.000 0.000 26  1.000 0.000  −5 −1.000   0.000 27   0.000 0.000  −4 1.000 0.000 28  0.000 0.000  −3 1.000 0.000 29   0.000 0.000  −2 1.000 0.000 30  0.000 0.000  −1 1.000 0.000 31   0.000 0.000

Provided that samples in the time domain in the first 64-sample T₁ matchcorresponding samples in T₂ at the single-antenna device 110, the phasesand amplitudes of each subcarrier after an FFT of the samples in T₁ willalso match the phases and amplitudes of each subcarrier after an FFT ofthe samples in T₂. If the samples in the time domain do not match,however, the phases and amplitudes of the subcarriers will also notmatch.

The channel between the transmitter 120 and the single-antenna 110 maymodify the transmitted signal because the signal takes multiple pathswhile in flight, reflecting off or passing through objects in theenvironment. These multi-path signals add up constructively ordestructively at the single-antenna device 110, and the result is thatthe samples are not received with the same phase and amplitude withwhich they were transmitted. This signal change suggests the possibilitythat samples in T₁ may not have the same phase and amplitude as thecorresponding sample in T₂ when the signal is received. However, thedisclosure below demonstrates that those samples match or substantiallymatch (except for random noise) when the single-antenna device 110 isnot in a near-field region of the transmitter 120.

The channel between the transmitter 120 and the single-antenna 110 maybe modeled by Equation 1, below:

y[i]=Hx[i]+w[i]  (Equation 1)

In this equation, y[i] is the i-th received sample, H is the channelmatrix representing the changes to the signal caused by the channel,x[i] is i-th the transmitted sample, and w[i] is noise received withsample i. In a static environment (e.g., no moving objects), H is timeinvariant and causes the same shift in phase and amplitude for allsamples in x because all transmitted samples take the same multipathsfrom sender to single-antenna device 110. Neglecting noise, the resultis that sample y[i] still matches sample y[i+64] in phase and amplitude,even though they no longer match x[i] due to the effects of H.

This phase and amplitude change in the received sample compared with thetransmitted sample is normal for wireless communication and is one ofthe reasons why Wi-Fi uses a preamble. The phase and amplitude of thepreamble samples are pre-defined by the Wi-Fi specification and areknown to both the transmitter 120 and the single-antenna 110. Thetransmitter 120 sends the preamble at the predefined phase and amplitudeand the single-antenna device 110 uses these known phase and amplitudevalues in the STF to detect the start of the frame and apply a coarsefrequency correction. Next it uses the LTF to synchronize symbol timingand apply fine frequency correction. Finally, because each subcarriermay be impacted differently by the channel, the single-antenna device110 performs an FFT of the received time-domain signal to independentlymeasure the phase and amplitude of each frequency-domain subcarrier inthe LTF. The single-antenna device 110 computes the difference from theknown transmitted phases and amplitudes for each subcarrier (see FIG. 3)and the received phases and amplitudes to estimate the channel's impacton each subcarrier. This estimate is called Channel State Information orCSI. The single-antenna device 110 uses this estimate from the LTF tocorrect for the channel's effects.

In Equation 1, it is assumed that H is time invariant so correspondingsamples in T₁ and T₂ will be received with identical phase and amplitude(except for noise). In some scenarios, however, the transmitter 120, thesingle-antenna 110, or other objects may be moving and that movement mayimpact the signal. A channel is said to be coherent if it is stable overa particular time interval. If the channel is coherent over a coherencetime, T_(c), for the corresponding portions of the preamble, then thecorresponding samples will be received with the same phase andamplitude. For example, assuming Wi-Fi samples at 20 MHz, meaning ittakes 20 million samples per second, the time for one sample, Ts, isthen 1/(20,000,000 samples/second), which equates to 50 ns. T₁ and T₂are a total of 128 samples long, and the coherence time T_(c) fordetermining whether T₁ matches T₂ is 6.4 μs (50 ns/sample×128samples=6.4 μs). That is, if the channel is stable over 6.4 μs, then T₁will match T₂ (aside from noise).

Moving objects can potentially cause a mismatch by changing the lengthof the signal's path as it travels from the transmitter 120 to thesingle-antenna device 110. The length of the path affects the phase andamplitude of the signal according to Equation 2, below:

$\begin{matrix}{H = {\sum\limits_{p = 1}^{P}\;{a_{p}e^{{- {j2d}_{p}}/\lambda}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

In this equation, j is the square root of −1, α_(p) is the attenuationof the signal along the path p, d_(p) is the length of path p, P is thenumber of paths, and A is the signal's wavelength. The length of path pmay change as the transmitter 120, the single-antenna device 110, ormultipath-inducing objects move. To cause a significant change in thesignal between corresponding samples, however, the movement would needto cause a change in path length of more than one-quarter wavelength(and one-half wavelength to cause maximum change). In Wi-Fi's 2.4 GHzband, the wavelength λ is approximately 12 cm, suggesting that an objectwould need to move approximately λ/4≈3 cm in 6.4 μs to significantlyimpact the phase and amplitude between corresponding LTF samples. Thistranslates to a speed of over 17,000 km/hour (and roughly twice thisspeed for Wi-Fi's 5 GHz band). Given the extraordinary speed an objectwould need to be moving to cause a substantial change in path length inthe short coherence time needed for the preamble, changing path lengthsis eliminated as a possible explanation for corresponding LTF samples tohave different phases and amplitudes.

FIG. 4 illustrates a plurality of regions surrounding a transmittingantenna. In this illustrated example, the transmitting antenna mayexemplify the at least one antenna (not illustrated) included in thetransmitter 120 of FIG. 1. The plurality of regions surrounding thetransmitting antenna include: (1) the reactive near-field, which isclosest to the transmitting antenna, (2) the radiating near-field, whichbegins after the reactive near-field, and (3) the far-field, whichbegins after the radiating near-field and extends to infinity. It shouldbe appreciated that the boundaries between the regions are not sharp,but instead transition gradually.

FIG. 5 illustrates the orientation of the transmitting antenna of FIG. 4in a three-dimensional space and a signal propagating from thetransmitting antenna of FIG. 4. In this illustrated example, it isassumed that the transmitting antenna is aligned vertically with the zaxis. The magnetic fields H of the signal relative to each axis aredetermined by Equations 3a and 3b, below:

$\begin{matrix}{H_{r} = {H_{\theta} = 0}} & \left( {{Equation}\mspace{14mu} 3a} \right) \\{H_{\phi} = {j{\frac{{\kappa I}_{0}l_{t}\sin\theta}{4{r}}\left\lbrack {1 + \frac{1}{jkr}} \right\rbrack}e^{- {jkr}}}} & \left( {{Equation}\mspace{14mu} 3b} \right)\end{matrix}$

The electric fields E are determined by Equations 4a, 4b, and 4c, below:

$\begin{matrix}{E_{r} = {\eta{\frac{I_{0}l_{t}{\cos\theta}}{2{r}}\left\lbrack {1 + \frac{1}{jkr} - \frac{1}{({kr})^{2}}} \right\rbrack}e^{- {jkr}}}} & \left( {{Equation}\mspace{14mu} 4a} \right) \\{E_{\theta} = {j\eta{\frac{{KI}_{0}l_{t}\sin\theta}{4{r}}\left\lbrack {1 + \frac{1}{jkr} - \frac{1}{({kr})^{2}}} \right\rbrack}e^{- {jkr}}}} & \left( {{Equation}\mspace{14mu} 4b} \right) \\{E_{\varnothing} = 0} & \left( {{Equation}\mspace{14mu} 4c} \right)\end{matrix}$

In these equations, j=k=√{square root over (−1)}/λ, is the wavenumber,I₀ is current applied to the transmitter 120, l₁ is the length of thetransmitting antenna, η=120π is the intrinsic impedance of free space, θis the vertical angle between the transmitter 120 and the single-antennadevice 110, ϕ is the horizontal angle between the transmitter 120 andthe single-antenna 110, and r is the distance extending radially fromthe transmitter 120.

Returning to FIG. 4, the reactive near-field region is the regionclosest to the transmitting antenna, where kr<1 (or equivalently, wherer<λ/2π). In this region, the reactive (e.g., non-radiating) fielddominates and there is a high content of non-propagating stored energy.Here, the wavefront is not spherical because the electric and magneticfields are not yet aligned, and in addition to the radiated energydescribed by the first term in brackets in Equations (3b), (4a), and(4b), there is a great deal of stored, non-propagating energy becausethe second and third terms inside the brackets dominate at close range.

With real antennas, the reactive near-field region is commonly estimatedto extend from the surface of the antenna to roughly R₁, wherein R₁ isdefined by Equation 5, below:

R ₁=0.62√{square root over (D ³/λ)})  (Equation 5)

In this equation, D=I_(t)+I_(r) is combined length of the transmittingantenna, l_(t), and the receiving antenna, l_(r), and λ is the signalwavelength. In some examples, with Wi-Fi 2.4 GHz band, andquarter-wavelength dipole antennas, this region extends to roughly 2.7cm from the transmitter 120. In some examples, with Wi-Fi's 5 GHz band,this region extends to roughly 1.1 cm.

The radiating near-field region is an area between the reactivenear-field and far-field regions. In this region, kr>1 and the electricand magnetic fields are predominantly in phase, but the wavefront isstill not yet spherical as it is in the far-field region. In view ofEquations 3b and 4a, unlike in the reactive near field, the first termin the brackets (i.e., “1”) begins to dominate the second term (i.e.,“1/jkr”) because kr is greater than one. Likewise, in Equation 4b, thefirst term in the brackets (i.e., “1”) begins to dominate the second(i.e., “1/jkr”) and third terms (1/(kr){circumflex over ( )}2). Becauseof the increasing value of kr compared with the reactive near-fieldregion, the energy in the radiating near field is largely real, that is,radiated energy.

Based on the magnetic fields H and the electric fields E, the averagepower of the signal, W, may be estimated based on Equation 6, below:

W=½(E×H*)  (Equation 6)

In this equation, * denotes complex conjugate and E and H are determinedusing Equations 3 and 4. W can be decomposed into its radial, Wr, andvertical, Wθ components as Equations 7a and 7b, below:

$\begin{matrix}{W_{r} = {\frac{\eta}{8}{\frac{I_{0}l_{t}}{\lambda}}^{2}{\frac{\sin^{2}\theta}{r^{2}}\left\lbrack {1 - {j\frac{1}{({kr})^{3}}}} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 7a} \right) \\{W_{\theta} = {j\eta{\frac{\left. k \middle| {I_{0}l_{t}} \middle| {}_{2}{\cos\theta\sin\theta} \right.}{16^{2}r^{3}}\left\lbrack {1 + \frac{1}{({kr})^{2}}} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 7b} \right)\end{matrix}$

FIG. 6 illustrates an example graph of power of the radial and verticalcomponents of a signal transmitted from the transmitter 120 to thesingle-antenna device 110. In this illustrated example, it is assumedthat the signal is transmitted via Wi-Fi's 2.4 GHz band withquarter-wavelength antennas. In the illustrated example, at distanceslarger than roughly 5 cm, the We component begins to dominate the Wrcomponent. At distances closer than about 5 cm, the radial component isstronger than the vertical component. This relative strength suggeststhat the power pulses inward and outward near the transmitter 120,whereas, at greater distances, the radial component dies out andvertical component takes over. This vertical component domination isindicative of signals in the far-field region, whereas radial componentdomination is indicative of signals in the radiating near-field region.

With real antennas, the radiating near-field region is commonlyestimated to extend from R₁ to R₂, where R₂ is defined by Equation 8:

R ₂=2D ²/λ  (Equation 8)

In this Equation, D=i_(t)+l_(r) is combined length of the transmittingantenna, l_(t), and the receiving antenna, l_(r), and λ is the signalwavelength. With Wi-Fi's 2.4 GHz band and quarter-wavelength dipoleantennas, Equation 8 suggests that the radiating near-field regionextends to approximately 6.2 cm from the transmitter 120. This estimateroughly matches the results shown in FIG. 6 using Equation 7, where thevertical component of the energy begins to dominate as it does it thefar-field.

The-far field is the area far from the transmitting antenna where kr>>1.Because kr is large in the far-field, several of the terms in Equations3 and 4 become extremely small and the E and H fields can beapproximated by Equations 9a, 9b, and 9c, below:

$\begin{matrix}{E_{\theta} \simeq {j\eta\frac{{kI}_{0}l_{t}e^{{- j}kr}}{4{r}}{\sin\theta}}} & \left( {{Equation}\mspace{14mu} 9a} \right) \\{{E_{r} \simeq E_{\theta}} = {H_{r} = {H_{\theta} = 0}}} & \left( {{Equation}\mspace{14mu} 9b} \right) \\{H_{\theta} \simeq {j\frac{{kI}_{0}l_{t}e^{{- j}kr}}{4{r}}{\sin\theta}}} & \left( {{Equation}\mspace{14mu} 9c} \right)\end{matrix}$

In Equations 9a, 9b, and 9c, the electric and magnetic fields arealigned orthogonal to each other (e.g., θ is orthogonal to ϕ),transverse to the direction of propagation, and are in timesynchronization. This alignment creates a spherical wavefront withaverage power given by Equation 6.

At ranges closer than roughly R2, the overall E and H fields are not inphase with respect to time, and because those fields do not have equalmagnitude, they form a vector that rotates in time in a plane parallelto the direction of propagation, rather than the stable orthogonalrelationship in the far-field region. As such, using such properties ofchange in phase and amplitude in the near-field region and the far-fieldregion, the single-antenna device 110 may determine whether the receivedsignal is transmitted from a trusted source or an adversary. Details inwhich the single-antenna device 110 renders such determination will bedescribed with example embodiments below.

Returning to FIG. 1, once the single-antenna device 110 receives thesignal including at least one Wi-Fi frame, the single-antenna device 110may determine whether the received signal is provided from a trustedsource (e.g., the transmitter 120). Example embodiments in which thesingle-antenna device 110 renders said determination will be describedin detail below.

In certain embodiments, once the single-antenna device 110 receives thesignal including at least one Wi-Fi frame, the single-antenna device 110calculates a total Euclidean distance between the phase and amplitude ofsubcarriers in the two 64-sample OFDM symbols T₁ and T₂ of the LTF. Thetotal Euclidean distance may be calculated based on Equation 10, below:

$\begin{matrix}{E_{j} = {\sum\limits_{k = {- 32}}^{31}\begin{bmatrix}{\left( {{\left( {Y_{1}\lbrack k\rbrack} \right)} - {\left( {Y_{2}\lbrack k\rbrack} \right)}} \right)^{2} +} \\\left( {{\left( {Y_{1}\lbrack k\rbrack} \right)} - {\left( {Y_{2}\lbrack k\rbrack} \right)}} \right)^{2}\end{bmatrix}^{\frac{1}{2}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

In this equation, E₁ is the total Euclidean distance between the phaseand amplitude of all subcarriers k for frame j, and where Y₁ is theresult of an FFT over T₁ and Y₂ is the result of an FFT over T₂,

(Y_(x))[k]) is the real component and ℑ(Y_(x) [k]) is the imaginarycomponent of each subcarrier k in Y_(x), for x∈{1, 2}. Herein, thisdifference E₁ is the preamble deviation of a frame. If the subcarriersin the two corresponding portions of the LTF are substantially the same,the preamble deviation may be small, whereas, if the subcarriers aredifferent in the two corresponding portions of the LTF, then thepreamble deviation is large.

In certain embodiments, once the single-antenna device 110 calculatesthe preamble deviation of a frame, it compares the same to a threshold,τ. If the single-antenna device 110 determines that the preambledeviation for a frame is greater than τ, the single-antenna devicedeclares proximity and determines that the received signal istransmitted from a trusted source. Otherwise, it does not declareproximity and determines that the received signal is transmitted from anuntrusted or adversarial source.

FIG. 7 illustrates an example constellation diagram showing the distancebetween Y₁ and Y₂ for a subcarrier. Specifically, the constellationdiagram shows the distance between Y₁ and Y₂ for a subcarrier of oneframe when the transmitter 120 is located at 6 cm from thesingle-antenna device 110 and for the subcarrier of another frame sentfrom 30 cm. In this illustrated example, Y₁ matches or substantiallymatches Y₂ at 30 cm, but at 6 cm, Y₁ does not match Y₂ due to near-fieldeffects as discussed above with reference to FIGS. 4-6.

FIG. 8 illustrates example constellation diagrams showing the distancebetween Y₁ and Y₂ for all subcarriers of one frame. The right exampleconstellation diagram illustrates Y₁ and Y₂ for all subcarriers at 30cm, and the left example constellation diagram illustrates Y₁ and Y₂ forall subcarriers at 6 cm. At 30 cm, Y₁ and Y₂ match or substantiallymatch for all subcarriers, but at 6 cm, many subcarriers do not match.

FIG. 9 illustrates an example distribution graph of preamble deviationsfor 1,000 Wi-Fi frames received from the transmitting antenna. In theillustrated example, the line in the box indicates the median value, thebox indicates the 75^(th) and 25^(th) percentile, and the whiskersindicate the maximum and minimum value. For this illustrated example, itis assumed that the transmitting antenna is a half-wavelength dipoleantenna. In this illustrated example, E_(j) at close range is typicallylarge but varies due to near-field effects. At long range (greater thanabout 12 cm) the preamble deviation is small and has much lower variancebecause the near-field effects have attenuated to near zero, as modeledin Equations 3 and 4. For brevity, the distributions from other types oftransmitting antennas have been omitted, however, it should beappreciated that they follow a similar pattern, with each having smallpreamble deviations and low variability beyond about 12 cm.

In certain embodiments, the single-antenna device 110 may furthercalculate an average preamble deviation over a number of frames for agiven antenna based on Equation 11, as provided below:

$\begin{matrix}{A_{t} = {\frac{1}{n}{\sum_{j = 1}^{n}E_{j}}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

In this equation, t is the type of antenna used to send Wi-Fi frames,and n is the number of frames received.

FIG. 10 illustrates an example graph of average preamble deviations overa plurality of frames transmitted over a plurality of distances for eachof a plurality of transmitting antenna types. In this illustratedexample, the plurality of frames are 1,000 frames, the transmittingantenna types include half-wavelength dipole, quarter-wavelength dipole,micropatch, and Panda Wireless® Panda Ultra Wireless N USB Adapter, andthe plurality of distances range from 2 cm to 300 cm. As modeled in thedisclosure above with reference to FIGS. 4-6, large average preambledeviations occur at short range (e.g., 2-8 cm), and small averagepreamble deviations occur at distances beyond roughly 12 cm. Thisrelationship holds across all of the transmitting antenna types andindicates that a single-antenna device 110 is operable to monitor theaverage preamble deviation and declare proximity when the averagepreamble deviation rises above a predetermined threshold. While thisillustrated example demonstrates change in average preamble deviationsbased on certain types of transmitting antennas, it should beappreciated that this relationship further holds across any other typesof transmitting antennas.

In certain embodiments, the threshold i may be set to 0.2 (as indicatedby the dashed line in FIG. 10) or around 0.2. since none of the averagepreamble deviations are over the threshold for any of the transmittingantenna types at distances over 14 cm, setting the threshold to 0.2ensures that the single-antenna device 110 does not falsely declareproximity when the transmitter 120 is far away.

In some situations, if the single-antenna device 110 uses only one frameto determine proximity, it could be the case that said frame happens tohave a low preamble deviation as indicated by the whiskers in FIG. 9,and the single-antenna device 110 may fail to recognize proximity eventhough it should. Such situations suggest that proximity detection maybenefit from measuring the preamble deviation from multiple framesbefore declaring proximity. Therefore, in certain embodiments, insteadof relying on the preamble deviation from a single frame, thesingle-antenna device 110 may: (1) calculate the average preambledeviation based on two or more frames; (2) compare that average valuewith the threshold τ; and (3) declare proximity based on the comparison.FIG. 11 demonstrates the benefit of using multiple frames for declaringproximity.

FIG. 11 illustrates an example graph of the likelihood of detectingproximity using the average preamble deviations. The example graph iscreated via a Monte Carlo simulation. In the Monte Carlo simulation, anumber of frames from the 1,000 Wi-Fi frames captured at each distancebetween the transmitter 120 and single-antenna device 110 is randomlysampled, and an average preamble deviation is calculated over thosenumber of frames. Specifically, the example graph represents thelikelihood of declaring proximity from 1,000 runs of the Monte Carlosimulation that randomly selected n∈{1, 2, 5, 10, 20} Wi-Fi frames ateach distance with τ=0.2. The results shown are the average over all ofthe transmitting antenna types, as described with reference to FIG. 10.In this illustrated example, the likelihood of declaring proximity ishigh when the transmitter 120, regardless of antenna type, is withinabout 9 cm (i.e., the effective range of preamble detection) and whenthe single-antenna device 110 uses more than one frame. Using more thantwo frames results in improved detection probability, however, theamount of improvement decreases as the number of frames used increases.

In certain embodiments, if the single-antenna device 110 declaresproximity and determines that the received signal is transmitted from atrusted source, the single-antenna device 110 may accept the data of thecurrent frame and/or that of the other frame(s) in the received signal.If the single-antenna device 110 does not declare proximity anddetermines that the received signal is transmitted from an untrusted oradversarial source, the single-antenna device 110 may reject the data ofthe current frame and/or that of the other frame(s) in the receivedsignal.

In some situations, the adversarial communication device 130 may becapable of transmitting a malformed preamble where T₁ does not match T₂in an attempt to trick the single-antenna device 110 into falselydeclaring proximity. To overcome an adversary transmitting malformedpreambles, the single-antenna device 110 may communicate with acommunication device having a pre-existing trusted relationship with thesingle-antenna device 110.

FIG. 12 illustrates another example system 1200 comprising a pluralityof communication devices. The plurality of communication devices includethe single-antenna device 110, the transmitter 120, the adversarialcommunication device 130, and a trusted communication device 140. Thetrusted communication device 140 may include at least one antenna,memory, and processor. In the illustrated example, the trustedcommunication device 140 has a pre-existing trusted relationship withthe single-antenna device 110. In this illustrated example, the trustedcommunication device 140 may be a Wi-Fi router. It should be appreciatedthat the trusted communication device 140 may be other types ofcommunication devices. In the illustrated example, the trustedcommunication device 140 is positioned further away from thesingle-antenna device 110 than the transmitter 120.

In certain embodiments, once the single-antenna device 110 determinesthat the preamble deviation of the repeating portion for one or moreframes of the received signal is greater than τ, the single-antennadevice 110 may determine whether the trusted communication device 140 islocated equal to or greater than a predetermined distance apart from thesingle-antenna device 110 by communicating with the trustedcommunication device 140. The predetermined distance may be two timesthe effective range of the preamble detection to rule out a legitimatetransmitter 120 positioned in between the single-antenna device 110 andthe trusted communication device 140. If the single-antenna device 110determines that the trusted communication device 140 is located equal toor greater than the predetermined distance apart from the single-antennadevice 110, the single-antenna device 110 may request a response fromthe trusted communication device 140 as to whether the trustedcommunication device 140 sees a matching preamble from the source thattransmitted the received signal. If the preamble is purposely malformed,both the single-antenna device 110 and the trusted communication device140 will see the high preamble deviation. Therefore, if the responseindicates that the preamble deviation of the repeating portion for oneor more frames of a signal received at the trusted communication device140 is greater than τ, the single-antenna device 110 may determine thatthe received signal is transmitted from the adversarial communicationdevice 130 and reject the received signal, and if the response indicatesthat the preamble deviation of the repeating portion for one or moreframes of the signal received at the trusted communication device 140 isgreater than τ, the single-antenna device 110 may determine that thereceived signal is transmitted from the adversarial communication device130 and reject the received signal.

In some situations, a trusted device may not be readily available withinproximity of the single-antenna device 110. In certain embodiments, thesingle-antenna device 110 may examine the strength of the receivedsignal when it detects a high preamble deviation. Since signal strengthdrops with the square of distance, a distant adversary will need totransmit a high-power signal for the single-antenna device 110 toreceive it with the same strength as a signal from a legitimate devicelocated a few centimeters away. To prevent the distant adversary fromtricking the single-antenna device 110 into believing that the malformedpreambles are legitimate signals from a nearby device, thesingle-antenna device 110 can measure the signal strength of frames withhigh preamble deviations and reject frames with a signal strength belowa threshold.

In some situations, proximity is necessary, but may not be a sufficientindicator of trust. In many cases, a user may not want his/her devicesto pair with other devices that are physically close. For example, in acrowded subway, people may be packed together tightly. Any devices theywear or carry may then come into unintended proximity with otherdevices. In those use cases, where devices may encounter untrusteddevices, the single-antenna device 110 may perform proximity detectiononly when the user provides an input (e.g., physical input or voicecommand), rather than blindly trusting nearby devices. Proximitydetection used in conjunction with user intent may help prevent distantadversaries from tricking legitimate devices into accepting maliciousframes.

In certain embodiments, in response to receiving a user input forinitiating the proximity detection process, the single-antenna device110 may provide an instruction (e.g., either visually via a displaydevice or audibly via a speaker) for a user to place the transmitter 120in a close proximity with the single-antenna device 110 for establishingsecure short-range information exchange. In certain embodiments, thesingle-antenna device 110 may wait for a predetermined amount of timeafter providing the instruction.

While the example embodiments described above exemplify proximitydetection based on Wi-Fi communication protocol, it should beappreciated that proximity detection is further applicable to othercommunication protocols that include a repeating portion. For example,said communication protocols may include Zigbee, Bluetooth, BluetoothLow Energy, etc.

FIGS. 13A and 13B illustrate an example flowchart of a method forestablishing secure short-range information exchange between asingle-antenna device and a transmitting device.

At block 1302, the single-antenna device determines whether a user inputhas been provided for initiating proximity detection. If so, the methodcontinues to block 1304. Otherwise, the method terminates.

At block 1304, the single-antenna device determines whether a signalcomprising at least one Wi-Fi frame has been received. If so, the methodcontinues to block 1306. Otherwise, the method returns to block 1304.

At block 1306, the single-antenna device calculates the preambledeviation for one or more frames of the received signal. Alternatively,the single-antenna device may determine an average preamble deviationbased on two or more frames of the received signal.

At block 1308, the single-antenna device determines whether the preambledeviation of one or more frames of the received signal is greater than athreshold τ. If so, the method continues to block 1310. Otherwise, themethod continues to block 1320. Alternatively, the single-antenna devicemay compare average preamble deviation for two or more frames of thereceived signal with the threshold τ. If so, the method continues toblock 1310. Otherwise, the method continues to block 1320.

At block 1310, the single-antenna device determines whether a trustedcommunication device is disposed at or greater than a predetermineddistance apart from the single-antenna device. If so, the methodcontinues to block 1312. Otherwise, the method continues to block 1316.

At block 1312, the single-antenna device requests the trustedcommunication device to examine a signal transmitted from the samesource.

At block 1314, the single-antenna device receives a response from thetrusted communication device and determines whether the trustedcommunication device indicates that the preamble deviation of one ormore frames of the signal received at the trusted communication deviceis greater than the threshold τ. If so, the method continues to block1320. Otherwise, the method continues to block 1318. Alternatively, thesingle-antenna device receives the response from the trustedcommunication device and determines whether the trusted communicationdevice indicates that an average preamble deviation of two or moreframes of the signal received at the trusted communication device isgreater than the threshold τ. If so, the method continues to block 1320.Otherwise, the method continues to block 1318.

At block 1316, the single-antenna device determines whether the signalstrength of one or more frames of the received signal is greater than athreshold. If so, the method continues to block 1318. Otherwise, themethod continues to block 1320.

At block 1318, the single-antenna determines that the received signal istransmitted from a trusted source and accepts one or more frames of thereceived signal.

At block 1320, the single-antenna determine that the received signal istransmitted from an adversary and rejects one or more frames of thereceived signal.

The flowchart of FIG. 13 is representative of machine-readableinstructions stored in memory (such as the memory 116 of FIG. 1) thatare executable by a processor (such as the processor 114 of FIG. 1).Although the example program(s) is/are described with reference to theflowchart illustrated in FIGS. 13A and 13B, many other methods mayalternatively be performed. For example, the order of execution of theblocks may be changed, and/or some of the blocks described may bechanged, eliminated, or combined.

In this application, the use of the disjunctive is intended to includethe conjunctive. The use of definite or indefinite articles is notintended to indicate cardinality. In particular, a reference to “the”object or “a” and “an” object is intended to denote also one of apossible plurality of such objects. Further, the conjunction “or” may beused to convey features that are simultaneously present instead ofmutually exclusive alternatives. In other words, the conjunction “or”should be understood to include “and/or”. As used here, the terms“module” and “unit” refer to hardware with circuitry to providecommunication, control and/or monitoring capabilities. “Modules” and“units” may also include firmware that executes on the circuitry. Theterms “includes,” “including,” and “include” are inclusive and have thesame scope as “comprises,” “comprising,” and “comprise” respectively.

The above-described embodiments, and particularly any “preferred”embodiments, are possible examples of implementations and merely setforth for a clear understanding of the principles of the invention. Manyvariations and modifications may be made to the above-describedembodiment(s) without substantially departing from the spirit andprinciples of the techniques described herein. All modifications areintended to be included herein within the scope of this disclosure andprotected by the following claims.

What is claimed is:
 1. A non-transitory computer readable medium storinginstructions executable by at least one processor, the instructions,when executed by said processor, cause said processor to operate with asingle antenna to: receive a wireless signal comprising at least oneframe, each of said frame comprising a repeating portion; determine adifference of phase and amplitude of the repeating portion; anddetermine whether the wireless signal is transmitted from a trustedsource based at least in part on the difference of phase and amplitudeof the repeating portion.
 2. The non-transitory computer readable mediumof claim 1, wherein each of said frame comprises a preamble, and whereinthe preamble comprises the repeating portion.
 3. The non-transitorycomputer readable medium of claim 1, wherein each of said framecomprises a Long Training Field (LTF), and wherein the LTF comprises therepeating portion.
 4. The non-transitory computer readable medium ofclaim 1, wherein the repeating portion comprises: a first set ofsymbols; and a second set of symbols, wherein the second set of symbolsis a repeat of the first set of symbols.
 5. The non-transitory computerreadable medium of claim 4, wherein, to determine the difference of thephase and amplitude of the repeating portion, the instructions, whenexecuted by said processor, cause said processor to calculate a value,wherein the value corresponds to a difference between phase andamplitude of the first set of symbols and phase and amplitude of thesecond set of symbols.
 6. The non-transitory computer readable medium ofclaim 5, wherein the instructions, when executed by said processor,cause said processor to: compare the value to a threshold; responsive tothe value being greater than the threshold, determine that the wirelesssignal is transmitted from the trusted source; and responsive to thevalue being less than the threshold, determine that the wireless signalis transmitted from an adversary.
 7. The non-transitory computerreadable medium of claim 6, wherein the instructions, when executed bysaid processor, cause said processor to: responsive to determining thatthe wireless signal is transmitted from the trusted source, accept thewireless signal; and responsive to determining that the wireless signalis transmitted from the adversary, reject the wireless signal.
 8. Thenon-transitory computer readable medium of claim 1, wherein theinstructions, when executed by said processor, cause said processor to:calculate an average preamble deviation based on two or more framescomprised within the wireless signal; compare the average preambledeviation to a threshold; responsive to the average preamble deviationbeing greater than the threshold, determine that the wireless signal istransmitted from the trusted source; and responsive to the averagepreamble deviation being less than the threshold, determine that thewireless signal is transmitted from an adversary.
 9. The non-transitorycomputer readable medium of claim 1, wherein the wireless signal istransmitted from a transmitter, wherein the difference of phase andamplitude of the repeating portion is a first difference of phase andamplitude of the repeating portion, and wherein, the instructions, whenexecuted by said processor, cause said processor and the single antennato: responsive to the first difference of phase and amplitude of therepeating portion being greater than a threshold, communicate with atrusted communication device to determine whether the trustedcommunication device is located at or greater than a predetermineddistance apart from the single-antenna device; responsive to determiningthat the trusted communication device is located at or greater than thepredetermined distance apart from the single-antenna device, request thetrusted communication device to: receive, from the transmitter, thewireless signal; determine a second difference of phase and amplitude ofthe repeating portion; and compare the second difference to thethreshold; and responsive to receiving a response from the trustedcommunication device indicating that the second difference is greaterthan the threshold, determine that the wireless signal is transmittedfrom an adversary.
 10. The non-transitory computer readable medium ofclaim 1, wherein the threshold is a first threshold, and wherein, theinstructions, when executed by said processor, cause said processor andthe single antenna to: responsive to determining that the difference ofphase and amplitude of the repeating portion is greater than the firstthreshold, measure a wireless signal strength of said frame; responsiveto the wireless signal strength being greater than a second threshold,determine that the wireless signal is transmitted from the trustedsource; and responsive to the wireless signal strength being less thanthe second threshold, determine that the wireless signal is transmittedfrom an adversary.
 11. The non-transitory computer readable medium ofclaim 1, wherein said frame is at least one Wi-Fi frame.
 12. A methodcomprising: receiving, at a single-antenna device comprising a singleantenna, a wireless signal comprising at least one frame, each of saidframe comprising a repeating portion; determining a difference of phaseand amplitude of the repeating portion; and determining whether thewireless signal is transmitted from a trusted source based at least inpart on the difference of phase and amplitude of the repeating portion.13. The method of claim 12, wherein each of said frame comprises apreamble, and wherein the preamble comprises the repeating portion. 14.The method of claim 12, wherein each of said frame comprises a LongTraining Field (LTF), and wherein the LTF comprises the repeatingportion.
 15. The method of claim 12, wherein the repeating portioncomprises: a first set of symbols; and a second set of symbols, whereinthe second set of symbols is a repeat of the first set of symbols. 16.The method of claim 15, wherein the determining the difference of thephase and amplitude of the repeating portion comprises calculating avalue, wherein the value corresponds to a difference between phase andamplitude of the first set of symbols and phase and amplitude of thesecond set of symbols.
 17. The method of claim 16, further comprising:comparing the value to a threshold; responsive to the value beinggreater than the threshold, determining that the wireless signal istransmitted from the trusted source; and responsive to the value beingless than the threshold, determining that the wireless signal istransmitted from an adversary.
 18. The method of claim 17, furthercomprising: responsive to determining that the wireless signal istransmitted from the trusted source, accepting the wireless signal; andresponsive to determining that the wireless signal is transmitted fromthe adversary, rejecting the wireless signal.
 19. The method of claim12, further comprising: calculating an average preamble deviation basedon two or more frames comprised within the wireless signal; comparingthe average preamble deviation to a threshold; responsive to the averagepreamble deviation being greater than the threshold, determining thatthe wireless signal is transmitted from the trusted source; andresponsive to the average preamble deviation being less than thethreshold, determining that the wireless signal is transmitted from anadversary.
 20. The method of claim 12, wherein the wireless signal istransmitted from a transmitter, and wherein the difference of phase andamplitude of the repeating portion is a first difference of phase andamplitude of the repeating portion, the method further comprising:responsive to the first difference of phase and amplitude of therepeating portion being greater than a threshold, communicating with atrusted communication device to determine whether the trustedcommunication device is located at or greater than a predetermineddistance apart from the single-antenna device; responsive to determiningthat the trusted communication device is located at or greater than thepredetermined distance apart from the single-antenna device, requestingthe trusted communication device to: receive, from the transmitter, thewireless signal; determine a second difference of phase and amplitude ofthe repeating portion; and compare the second difference to thethreshold; and responsive to receiving a response from the trustedcommunication device indicating that the second difference is greaterthan the threshold, determining that the wireless signal is transmittedfrom an adversary.
 21. The method of claim 12, wherein the threshold isa first threshold, the method further comprising: responsive todetermining that the difference of phase and amplitude of the repeatingportion is greater than the first threshold, measuring a wireless signalstrength of said frame; responsive to the wireless signal strength beinggreater than a second threshold, determining that the wireless signal istransmitted from the trusted source; and responsive to the wirelesssignal strength being less than the second threshold, determining thatthe wireless signal is transmitted from an adversary.
 22. The method ofclaim 12, wherein said frame is at least one Wi-Fi frame.